Such high-voltage test devices are sufficiently known from the prior art and are used for testing of measurement objects, which may be various electrical or electronic components or especially high-voltage or medium-voltage cables. For this purpose the measurement object to be tested is subjected to a usually sinusoidal a.c. voltage (with high amplitude higher than 100 kV) in the said power range and—accompanied by determination and evaluation of the test voltage applied to the measurement object as well as the test current induced thereby—an estimate, for example, is obtained as to whether the tested measurement object has withstood the test voltage for a specified time without voltage breakdown. Furthermore, it is also possible under certain circumstances to determine, for example by successively raising the test voltage, the test voltage from which voltage breakdowns relevant for safety occur or have occurred. Moreover, especially when the high-voltage test device of the class in question is set up to generate a preferably sinusoidal VLF (very low frequency) a.c. voltage with a frequency in the range between 0.01 Hz and 1 Hz, it is possible to determine, while at the same time evaluating the resulting phase shift between test voltage and test current, the so-called loss factor (tan δ) of the measurement object, with which it is possible, in the context of a nondestructive test, to obtain, for example, an estimate of the quality or of the aging condition of the insulation of high-voltage or medium-voltage cables. The measurement and evaluation algorithms to be applied in this respect (and the measurement and evaluation circuits necessary for this purpose) are sufficiently known to the person skilled in the pertinent art.
As explained in the introduction, this test voltage is frequently generated in the prior art by using two voltage-amplifier branches integrated into a high-voltage test device of the class in question, of which one generates the positive voltage half waves and the other the negative voltage half waves of the test voltage, which can then be applied in appropriate alternating manner to the measurement object.
Especially in the case of VLF testing of high-voltage or medium-voltage cables, the requirements for the voltage amplitude and electrical power to be applied by test devices of the class in question become particularly stringent with increasing length of the cable to be tested, and so a great need exists for test devices with correspondingly high power. Furthermore, it is of importance in the scope of the present invention that high-power high-voltage test devices of the class in question usually must be transported to the measurement object to be tested, which is not always simply possible for devices known from the prior art.
A present, mobile (VLF) high-power high-voltage test devices of the type mentioned in the introduction are available with means for generating a sinusoidal VLF test voltage with an amplitude of up to 200 kV (corresponding to approximately 141 kV rms voltage) and electrical output powers of up to approximately 8 kW.
Obviously the costs of (mobile) high-power high-voltage test devices of the class in question are determined definitively by the output power to be applied by the device in question, and so, in practice, high-voltage test devices of the type mentioned in the introduction are usually offered in various power classes, the operational capabilities of which—depending on the voltage amplitudes that can be generated with the test devices in question and on the associated available output power—are limited.
And, finally, it has been found, especially when a high output power must be made available in high-voltage test devices, that the necessary cooling of the high-voltage electronics is becoming increasingly problematic, especially for the components of the various voltage-amplifier branches disposed therein on the high-voltage side. This is the case in particular when the said devices must be transported as mobile test devices to the measurement object, which in the prior art is frequently implemented by providing a device structure that is as compact as possible.